During the drilling of wells, a drilling fluid is circulated down the drill pipe past the teeth of the cutter head to sweep away the rock cuttings from the cutter head, and move them to the surface. It is essential that the cuttings be carried upwardly and out of the bore by a fluid in order that the cutter will not become clogged, and so that the drilling operation can continue for extended periods of time. The fluid that is usually used is water to which certain materials are added to produce a slurry which better lubricates the cutter head, and which has a higher specific gravity than water, so that it is better able to lift the rock cuttings to the surface.
In order to prevent the settling out of the rock cuttings in the bore, should flow stop for some reason, it is highly desirable that the slurries have thixotropic properties whereby they increase in viscosity or "gell" when flow is stopped, and thin out again when flow is again initiated. One material that is commonly used to provide such thixothrophic properties is clay. Clay type drilling slurries are called "drilling muds" and typically have a specific gravity of 1.3, and a flow viscosity at 20.degree. C. of approximately 20 centipoise. One commonly used method of determining the flow viscosity of drilling fluids is called the Yield Point Method wherein the difference in torque of the inner bob of a direct indicating Viscometer at 300 RPM and 600 RPM is subtracted from that at 300 RPM. The torque so extrapolated to zero RPM is called the "Yield Point."
During the drilling of wells, it is a common occurance for the cutter head to pass through porous layers of gravel or rock which will not retain the drilling mud; so that circulation of the drilling mud is lost. "Muds" may contain still other special additives such as barium salts to further increase their specific gravity. With or without special additives, drilling muds are sufficiently expensive that they must be continuously recirculated. The muds are brought to the surface, the rock cuttings removed, and the muds are pumped back down the well. Seepage of drilling muds into porous strata may plug off oil bearing layers, or gas producing layers to make the well being drilling, worthless, and this is another reason why their loss must be prevented.
The art has long added materials to drilling muds for the purpose of sealing off porous subterranian layers through which the drilling cutters pass. It has been said that "everything under the sun" has been tried as a "lost circulation additive" to seal off subterranian porous layers. As far as we are aware, there has never been a set of criterion developed whereby the effectiveness of a potential sealing material can be predicted before testing. The America Petroleum Institute has developed a testing procedure that is set forth in their standard R. P. 13B Section 10 entitled: Standard Procedure For Testing Drilling Fluids for testing under conditions which simulate the kinds of porosity that exists in subterranean strata. The A. P. I. apparatus comprises a vertical chamber approximately 73 mm in diameter at the bottom for supporting a marble bed approximately 57 mm high. The bottom of the chamber has an outlet in which slotted plates can be inserted and removed to check the effectiveness of the material relative to cracks of various widths. A sample of three thousand five hundred cubic centimeters of drilling mud containing a potential sealing additive is poured into the vertical column on top of the marble bed, and the slurry is allowed to drain by gravity until a seal occurs. The amount of effluent is measured, and thereafter a gas pressure is gradually applied on top of the liquid. This gas pressure is containually increased and the amount of effluent is measured when a seal occurs. The seals may be blown out and a new one formed, one or more times, until a maximum of 1000 psi (70.3 KGF per sq. cm.) is developed in the column. For the test results tabulated below, marbles having a uniform diameter of 14.3 mm were used. In the slot test results tabulated below, the marbles were removed so that free fluid access to the slot plate existed. The slot used in the tests had a width of 5 mm except for the tests in Examples 13, 14 and 15 in which a slot of 3 mm was used.
The prior art is replete with test results that have been made of all kinds of lost circulation additives, in either actual wells, or in screening tests using the above described A. P. I. testing procedure. These materials have ranged from straw, nutshells, ground whole corncobs, and dough balls, to strips of rubber or cellophane. It will be understood that a maximum of additive particle size which can be used occurs because of the nozzle sizes which are used in the cutter heads to direct the drilling mud at the cutter teeth. The nozzle sizes used may be as small as 8 mm and so it is highly desirable that the particle size of the additive not be larger than 8 mm so that the additive can be universally used. The nature of the problem will now be apparent, since it is necessary to seal openings using particles having a diameter that is less than, or only slightly larger than the width of the hole which the particles are intended to seal. It can now be seen why the art has tried to use straw and strips of cellophane having a length greater than 2.54 cm; and it will further be apparent that the use of such materials can create plugging and other problems in the recirculating equipment for the muds.
As previously explained, drilling muds are thixotropic and will have a yield point which generally cannot be exceeded without creating problems. It is highly desirable, therefore, that the drilling muds devoid of lost circulation additives, and which are continually recirculated will themselves approach this yield point, in order that the drilling mud will have as great a rock chip suspending capability as is possible during normal operation. This being true, it is highly desirable that the lost circulation material additive have no appreciable effect on the yield point of the drilling muds. Most of the materials that have been tried heretofore including ground corncobs, greatly increased the yield point of the drilling muds, and therefore create problems when used. Ground walnut shells do not greatly increase the yield point and so have been used extensively. Ground walnut shells, however, have size limitations, are in limited supply, and are relatively expensive.
With this background, it is an object of the present invention to provide a more effective lost circulation additive which will not greatly affect the yield point of drilling muds.
Another object of the present invention is the provision of a lost circulation additive of the above described type, which can be obtained from common, and inexpensive, and readily available materials, and so is more economical than the prior art materials.
Further objects and advantages of the invention will be apparent to those skilled in the art to which the invention relates from the following description of applicants' preferred embodiments.